Coaggregation between S. gordonii PK488 and A. naeslundii PK606 is mediated by an adhesin of 38 kDa, which also may recognize salivary receptors. All streptococci that coaggregate with this actinomyces express an immunologically cross-reactive protein to this adhesin. From one of these streptococci, S. sanguis 12, the SsaB gene which encodes this adhesin has been cloned into E. coli and has been sequenced. It is 927 basepairs long, and encodes for a hydrophilic protein of molecular weight of 34,684 which has a hydrophobic signal peptide of 19 amino acids. Antisera against the cloned protein cross reacts with the 38 kDa protein from S. gordonii PK488. The ability of these streptococcal surface proteins to recognize both salivary receptors and actinomyces receptors provides the first example of a dual adhesive function of a surface protein recognizing both cellular and noncellular receptors. Coaggregation between V. atypica PK1910 and Streptococcus spp. appears to occur by two distinct adhesins, and one is galactoside-sensitive. Coaggregation-defective mutants that have lost the galactoside-sensitive coaggregation with streptococci also do not possess a protein of about 47 kDa. A protein of this size has been eluted from an N-acetylgalactosamine-agarose affinity column and is presumed to be the lactose-sensitive adhesin. While intergeneric coaggregation occurs with over 90% of the more than 700 human oral bacterial strains that we have examined, intrageneric coaggregation appears to be considerably more restricted. Only members of the genus Streptococcus and a few strains of Actinomyces exhibited intrageneric coaggregation. Both of these genera are initial colonizers of a clean tooth surface and intrageneric coaggregation may be of considerable importance in establishing the pioneer microbial community on a nascent surface. The results of each of these investigative approaches are focused on understanding the molecular basis of cell surface recognitions among oral bacteria and their role in microbial ecology.